The present invention relates to a heat transfer sheet. More particularly, the present invention has for its object to provide a heat transfer sheet capable of easily giving to a material to be transferred thereon a recording image excellent in color density, distinction and fastness properties.
Heretofore, there has been known various heat transfer systems including a sublimation type of transfer system in which a sublimable dye is carried as a recording medium on a base sheet such as paper to form a heat transfer sheet. The heat transfer sheet is put on a material to be transferred thereon and dyeable with a sublimable dye, for instance, a polyester woven fabric, and a pattern of heat energy is imparted from the back side of the heat transfer sheet to transfer the sublimable dye into the material to be transferred thereon.
In the above sublimation type of transfer system, the material to be transferred comprises a polyester woven fabric for instance. Accordingly, comparatively satisfactory transfer of the dye is achieved as a result of being heated by the heat energy imparted over a relatively extended period of time.
With advancements of recording systems, however, it is increasingly required to form small characters, patterns or photographic images on a material to be transferred. Such material comprises a dye receiving layer on, e.g., polyester sheet or paper. In this case, these images are formed at high speeds by exposure to heat energy with a thermal head for a very short length of time on the order of second or less. Within such a short length of time, it is thus impossible to form image of sufficient density, since the sublimable dye and the material to be transferred are not sufficiently heated.
To cope with such fast recording, therefore, sublimable dyes excellent in sublimability have been developed. However, problems with such sublimable dyes are that, owing to their low molecular weight, they may pass into the material or bleed on the material after printing, thus the already formed image is to become disturbed or unclear, or contamination surrounding articles is apt to occur.
In the case where a sublimable dye having a relative high molecular weight is used to avoid such problems, it is then impossible to form an image of such satisfactory density as mentioned, since the rate of sublimation drops in such fast recording systems as mentioned above.